[DAG] Fold fdiv X, c2 -> fmul X, 1/c2 without AllowReciprocal if exact (#93882)

This moves the combine of fdiv by constant to fmul out of an
'if (Options.UnsafeFPMath || Flags.hasAllowReciprocal()' block,
so that it triggers if the divide is exact. An extra check for
Recip.isDenormal() is added as multiple places make reference
to it being unsafe or slow on certain platforms.